THE ROLE OF DYNAMICS IN THE MACHINING PROCESS (MetalMAX TM Approach to Improving Milling Cutting...

THE ROLE OF DYNAMICS IN THE MACHINING PROCESS

(MetalMAXTM Approach to Improving Milling Cutting Performance)

Ideal Milling Process

Long Tool Life

Eliminationof benching

Optimum M/C utilizationLong Spindle Life

Max MRR/SGR

Unattended machining/High

Process reliability

Right first time

Stable Machining/

Low vibration

Low Cutting Forces

The Ideal Milling Process

Cutting Parameter SelectionHow do we choose our speeds, feeds and depths of cut

The Conventional Approach

• Highly Experienced Planner• Technological database from cutting tool supplier• Operational Guidelines from machine tool supplier• TATATA…J.Fox.1998

Note: None of the above is based on a sound scientific or objective approach.

Consequences of theConventional Approach

• Scrapped Parts• Excessive “benching”• Power tool life and tool failures• Accelerated spindle wear• Poor process reliability• Unpredictability

All of this results in wasted time and money

Common factor in the above trends is the increased importance of dynamic influences.

Trends exacerbate these problems

• Move to monolithic structures• Bigger,deeper parts with high L/D ratios.• Very Expensive, less margin for error.• Greater opportunity to shine

• Move to Flimsier, lightweight parts• Move to more exotic materials

How can we scientifically select the cutting parameters to account for the system

dynamics?• Quickly obtain required dynamic information• Use this information to obtain optimum cut

parameters • Quickly verify cutting performance.

What is High Speed Machining?

• There are many definitions• Cutting speed alone (tool maker viewpoint)• Spindle speed alone (common for newcomers)• Machining at speeds significantly higher than conventional practice (machine shop view)• Others

• All of the above definitions of high speed

machining are correct from someone’s point of

view

High Speed Machining (HSM) Definition

• From a dynamics perspective we define HSM as:

“High-speed machining occurs when the tooth passing frequency

approaches the dominant natural frequency of the system”

Professor Scott Smith, UNCC, Charlotte NC

The Role of Dynamics inHigh Speed Machining

• HSM is greatly influenced by the dynamic characteristics of the machine-tool-work piece system.

• In HSM, upper limits are denoted by onset of “chatter”.

• Success in HSM depends heavily on the ability to recognise and deal with dynamic problems.

• Selection of an appropriate spindle speed and depth of cut is extremely important and not obvious

Stability Lobe DiagramStability Lobe Diagram

ProcessDampingRegion

Chatter Mechanism• Most undesirable vibrations in milling are self-

excited chatter vibrations.

• What mechanism is responsible for transforming the steady input of energy (from the spindle drive) into a vibration?

• The primary mechanism is “Regeneration of Waviness”.

• The force on any tooth is proportional to the chip thickness

• Each tooth removes material from a surface generated by the passage of a previous tooth.

• Any vibration at the time that surface was being made results in a wavy surface.

bfKF s

Regeneration of Waviness

Process Damping• Chatter vibrations are inhibited at low speeds by

“process damping”.• Interference between the rake face of the tool and

the tool path produces a net damping force.• Dependent on surface velocity (spindle speed and

cutter diameter) and flexible frequencies of cutter.

Machine a part right the first time!

MetalMAXTM Hardware

The MetalMAX™ Approach

• Identify and isolate problems

areas• Predict dynamic behaviour• Adjust to optimise.

• Measure and verify• Optimised? - if not back to step

1• Move on

MetalMAX™The package for

dynamic/chatter prediction and controlFrequency and

Flexibility Measurement(Modal Analysis “Tap” Test)

+ Basic Cutting Parametersand

Cutting Theory=

Predictions of Stable Depth of Cut limitsCutting Forces and Displacements

Dynamic Cutting AccuracyELIMINATE CHATTER!!!

~

Measurement and Analysis

TXF PCScope

MilSim™

Frequency Analyserfor Machine Tools

Data Acquisition and Machining Analysis

Milling Simulation andChatter Prediction

Computation and Prediction

Verifying Performance

NC IntegratedSpindle Speed Control

Non Automated CRAC Package

FRF Measurement with MetalMAX™ Equipment

Schematic of Measurement Setupfor TXF “Tap” or “Ping” test.

Actual MetalMAX™ Equipment

4

3

2

1EXCITATION

(HAMMER)

RESPONSE

(ACCEL)

Sensor Interface Module

PC

Accelerometer

STRIKE

Hammer

Power Cable

Sensor Cable

FREQUENCY RESPONSEFUNCTIONS (FRF’S)

X-DIRECTION

Y-DIRECTION

Flexibility

INFORMATION NEEDEDTO GENERATE LOBING DIAGRAMS FROM FRFS

Tool geometry

CuttingParameters

Material Parametersare reduced to 2:Cutting StiffnessPD Wavelength

Material/Tool Specification

OrthogonalMeas. File

Cutting Limitations

Stability Lobe Plot20 mm 3-fluted Tool in 30 kW 24 krpm Spindle

ProcessDampingRegion

Unstable

Torque Limit

ChatterFrequencies

Power Lobe Plot20 mm 3-fluted Tool in 30 kW 24 krpm Spindle

Full Power

Modal Parameter Estimation

Natural FrequencyModal StiffnessModal Damping Ratio

Milling Simulation (Computer Model)

Cut Dataand info.

Data loaded from TXF

File

Milling Simulation (Results)

Stability Lobe Diagram

Power Lobe Diagram

Y-Displacement at 12,000 rpm

Y-Displacement at 11500 rpm

Chatter Frequency

Limitations of Approach• Critically dependent on cutting stiffness and process damping

wavelength.• Once established for a particular grind of tool and material then will

produce accurate predictability.• Will change after tool wears.

• 1/4” diameter tool is practical lower limit of effective measurement.• Improvements currently being developed• In worse case an indirect measurement approach can be applied.

• Measurement of dynamics performed under static conditions.• Measurements can be made at speed with non-contact sensor.• Most advance and current spindle designs have good dynamic

repeatability and consistency.

An Example of Benefit Obtained• Spar Mill Cutting with 1.25” Diameter indexable

mill with 2 inserts.• Initial Conditions (5 mm depth, max. full dia.):

• 21,500 rpm, 0.11 mm chip load, 118 mins. per load machining time.

• Getting chatter when cutter becomes fully immersed, lowered chip load to attenuate damage to part.

• New Conditions:• 24,000 rpm, .2 mm chip load, 62 mins. per load

machining time.

• Benefits• Savings: $35 per load.• Approximate 50% increase in machine capacity

(near 50% reduction in machining time per load).

Other Benefits of Easy Dynamic Measurement

• Rapid dynamic measurement can quickly identify many conditions.• Non-intuitive behavior.• Most flexible mode may not be the most likely to

chatter.• Quickly identify which component is producing the

most flexible mode.• Identify when stiffness or damping is loss.

• Quickly detect changes or compare performance.

Non-intuitive behavior: Shorter not always better.

FRF Stability Map

3 flute carbide 3/16” diameter ball-nose tapered end-mill with 5/8” shank6.9” overall length

3 flute carbide 3/16” diameter ball-nose tapered end-mill with 5/8” shank6.3” overall length

Most Flexible Mode May not Cause Chatter.

Long 1” Mill in Collet HolderStandard 3/4” Mill in SF Holder

Maximum Dynamic Flexibility

Critical for Chatter

Quickly identify Weak Component.

Spindle Side1 2 3

1-at tool tip 2-at tip of holder 3-at base of holder near spindle

Tool Mode

Holder Mode

Spindle Mode

1-at tool tip2-at tip of holder3-at base of holder near spindle

Detecting Problems after “Events”

Spindle loss bearingpreload. Subsequent measurements confirm that there was nopreload.

Same Tool and holderon two different machines,spindles of different age butstill in “good” condition.

• It determines whether chatter is or is not present.• It does this by “listening” to the cut and

suggesting alternative spindle speeds that harmonise the “good” and “bad” vibrations, producing constant chip thickness.

• Knowledge of the spindle speed is essential.• Spindle speed components generally dominate

the audio spectrum unless chatter is very severe.

• Other audio sources are related to spindle speed, bearing passing frequencies, air-oil hiss, etc.

• Correct setting of threshold maximizes sensitivity.

Trial and Error Example using Harmonizer®

10,000 RPM

Corner Cut raw audio signal.

10,000 RPM

Frequency content with filters

4 Fluted 25 mm diameter Carbide End-Mill in Collet holder with maximum speed of 10,000 rpm

Trial and Error Example

8393 RPM

Frequency content with filters.

8393 RPM

Corner Cut raw audio signal.

4 Fluted 25 mm diameter Carbide End-Mill in Collet holder with maximum speed of 10,000 rpm

Trial and Error Example

8393 RPM

Frequency content no filters.

10,000 RPM

Frequency Content with no filters.

4 Fluted 25 mm diameter Carbide End-Mill in Collet holder with maximum speed of 10,000 rpm

Tool Tuning

• With knowledge of the dynamics we can exploit the behaviour to our advantage.

• From a previous slide we know length is critical, sometimes shorter is not better.

• We can many times select holder and tool geometry to produce best performance at maximum speed.

Tool Tuning Example:30 kW, 24,000 RPM Spindle with 20 mm 3-Fluted tool

Full Power 30 kW12 mm depth of cut Not full Power 30 kW

4 mm depth of cut

70 mm stick-out 90 mm stick-out

Tests on KRYLE VMC

Damping trials

• CL and Particle damping tested• Harmonizer software used to record

sound levels

Stability Lobes: Undamped

Stability Lobes: Damped

Conventional Milling left to right;Particle damping, CLD

Un-damped 6000 rpm

CLD 6000 rpm

Webster & Bennett VTL

• Initial Spindle speed 30 rpm 3mm DOC• Tap Tests on Component, Ram & Tool• Deflection of Ram recorded during

turning• Excitation of Tool reduced by

increasing spindle speed

Webster & Bennett VTL

Tap Test Results• Four dominant modes identified from

tool; 870 Hz, 2500 Hz, 3500 Hz, 4500 Hz

• Accelerometer recordings during turning at 30 rpm show excitations at 3500 Hz and 4500 Hz

• Increasing the spindle speed to change the cutting frequency reduced the excitation at the tool tip

Webster & Bennett VTL

30 RPM

40 RPM

Presentation available on-line at:www.mfg-labs.comclick on “Download” to go to download area.